The present invention relates to a semiconductor integrated circuit, and more specifically to a read only memory circuit suitable for incorporation into an integrated circuit.
Generally, to achieve a high product yield, one or more redundant circuit blocks are provided for semiconductor integrated circuits, particularly for semiconductor memory devices. The redundant circuit blocks are usually formed in advance on a semiconductor chip to form an integrated circuit network with other circuit blocks. If a faulty circuit, a memory cell, for example, is found during tests performed on the fabricated integrated circuit, the faulty circuit is replaced by a redundant circuit having equivalent specialized functions, and thus, the desired comprehensive functions of the integrated circuit can be ensured.
In a conventional semiconductor integrated circuit equipped with such redundant circuit blocks, there is also a circuit having a fuse with which the circuit generates control signals to indicate the use of the redundant circuit blocks according to the ON or OFF state of the fuse.
FIG. 4 is an equivalent circuit diagram shown in an exemplary convention control signal generating circuit, a type of read only memory (ROM), including the fuse. Referring to FIG. 4, a fuse 51 is turned OFF by supplying an input signal B to the gate of an n-channel transistor 52. One end of the fuse 51 is connected to a voltage source Vcc (+5 volts, for example) and the other end of the fuse is connected to a node of a resistor 53 and the drain of the transistor 52. The control signal (output signal) C is provided from the node. When the fuse 51 is held ON, the voltage at the node is substantially equal to Vcc and the control signal C is at a high level. If the fuse 51 is OFF, the node voltage, i.e., the control signal changes to a low level.
The OFF state of the fuse is realized as follows: when the transistor 52 is turned ON by an input signal B of a predetermined voltage applied to the gate thereto, a large current flows through the transistor 52, and thus, the fuse 51 is blown by the current and thus becomes OFF.
The replacement of a faulty circuit in a semiconductor integrated circuit is described by taking a faulty memory on a memory chip as an example. If a faulty memory cell is detected during the chip test process, the fuses are turned OFF or remain ON to produce a binary information consisting of high level (1) and/or low level (0) signals, corresponding to the respective addresses of the row and column of the faulty cell. In the circuit of FIG. 4, a low level signal (0) is output if the fuse 51 is turned OFF, as mentioned above. During the access of the memory chip, the address input to the memory chip is compared with the information comprising the high and/or low level signals corresponding to the states of the fuses, and if a correspondence between the addresses and the information is detected, the row and column corresponding to the faulty cell is operatively disconnected from the memory circuit and the redundant circuit block is connected instead. Thus, a faulty circuit containing the faulty memory cell is replaced by the redundant circuit block.
To summarize the above, one or more fuses are used as a ROM for indicating the use of the redundant circuit block in a conventional semiconductor integrated circuit. The incoming address signals are compared with the combination of the ON or OFF states of the fuses and a determination of whether they correspond to the address signals corresponding to the faulty circuit or not is carried out. If the correspondence between the incoming address signals and the address of the faulty circuit is detected, the redundant circuit block is instructed to substitute for the faulty circuit.
However, an erroneous ON state sometimes occurs in a ROM having the above described fuses due to an insufficient blowing of the fuse by the electric current during the write-in operation. This is because the fuses are usually formed from thin film lines of polysilicon, for example, embedded in an insulating matrix such as a silicon dioxide layer. An insufficiently blown fuse could recover its conduction even if it had lost the conduction just after the write-in operation. Thus, a fuse, which should be made OFF, remains ON and produces erroneous information.
The destructive technique of blowing the fuse provides another kind of disadvantage such as producing cracks in the cover layer which is usually a glass layer formed on the surface of a semiconductor chip for the passivation purposes. This results in the failure of a semiconductor integrated circuit formed thereunder due to the absorption of moisture which penetrates through the cracks and gives deleterious influences to the circuit. Accordingly, the reliability of the ROM including fuses is insufficient for high quality integrated circuits, in general.